Who decided 1 wasn't prime

No single person "decided" that 1 isn't prime—rather, the mathematical community gradually adopted this convention through the 19th and early 20th centuries as number theory became more formalized. Historically, mathematicians held varying views: some ancient and medieval mathematicians considered 1 to be prime, while others did not. The modern definition crystallized because excluding 1 from the primes makes fundamental theorems cleaner and more powerful, particularly the Fundamental Theorem of Arithmetic, which states that every integer greater than 1 has a unique prime factorization.

If 1 were considered prime, this uniqueness would break down: the number 6 could be written as 2 × 3, or 1 × 2 × 3, or 1 × 1 × 2 × 3, creating infinitely many "different" factorizations. By defining primes as natural numbers greater than 1 with exactly two distinct divisors, mathematicians preserve the elegant structure of number theory and simplify countless theorems. This definitional choice represents mathematical consensus based on utility rather than an arbitrary decision—it's similar to how defining 0⁰ as 1 in certain contexts makes combinatorics cleaner, or how choosing specific axioms makes certain mathematical systems more productive. The convention that 1 is not prime became standard by the early 1900s, though you can still find older texts treating 1 as prime.